Interpretive Summary: The era of glyphosate-resistant crops (GRCs) began with a commercial launch of glyphosate-resistant (GR) canola and GR soybean in 1996. Within two years, two more GRCs (cotton and corn) were commercialized in the US. GRCs provide the flexibility to apply glyphosate (a non-selective herbicide) postemergence to control emerged weeds without concern for crop damage. The consistent and substantial weed control and economic benefits of GRCs have encouraged a rapid adoption by the US farmers. The remarkable commercial success of GRCs has impacted herbicide use. Glyphosate use has increased tremendously with a concomitant increase in selection pressure that promotes weed species shifts. Farmers’ love for GRC technology is being challenged by the looming threat of weed species shifts in GR cropping systems. Weed species shifts, a relative change in weed abundance, and species diversity in response to continuous use of glyphosate in GRCs are inevitable and are rapidly increasing (e.g., Benghal dayflower in GR cotton). Scientists at Southern Weed Science Research Unit, Stoneville, Mississippi and Department of Crop, Soil, and Environmental Science, University of Arkansas have summarized in this book chapter, the impact of GR corn, GR cotton, and GR soybean cropping systems on weed species shifts as well as late-season weed problems in the US.

Technical Abstract:
In 1996, transgenic GR canola and GR soybean containing a bacterial gene that imparts resistance to glyphosate were commercialized in the US. Later, GR cotton (1997) and GR corn (1998) were commercialized for planting in the US. GRCs enabled in-crop postemergence application of glyphosate. The effectiveness of glyphosate on a wide spectrum of weeds, simplicity and flexibility in application, lower herbicide cost, and freedom to rotate crops have encouraged a rapid adoption by the US farmers (Gianessi 2005, 2008; Reddy and Whiting 2000).
Because of efficient and consistent weed control and economic benefits, US farmers have continued to plant more area to GRCs each year. GR soybean, GR cotton, and GR corn are dominant among all other GRCs grown commercially in the US. The wide-spread adoption of GRCs coupled with a spectacular increase in glyphosate use has exerted tremendous selection pressure on weed communities. Over-reliance on glyphosate and inadequate diversity in weed management tactics in GRCs have resulted in weed species shifts. Weed species shifts refers to a relative change in weed population (abundance) or species (diversity) as well as late-season weeds in an agricultural system in response to weed management tactics. Weed species shifts in GRCs are a result of weeds that have escaped control because of a high level of tolerance to glyphosate or glyphosate avoidance from late-emerging cohorts. Common lambsquarters, johnsongrass, Italian ryegrass, Amaranthus, Ambrosia, Commelina, Cyperus, Ipomoea, and Setaria species are becoming problematic weeds in GRCs. Shifts in weed species can be prevented and shifts when does occur can be managed with prudent selection of weed control methods. Multiple herbicide-resistant (stacked traits) crops that combine glyphosate resistance with other herbicides and crops resistant to glufosinate can provide a wide range of foliar- and soil-applied herbicide options to manage weeds that survive glyphosate. Diversity in weed management systems is critical to reduce weed species shifts and to maintain sustainability of GRCs as an effective weed management tool.